Optical sensors are used in a number of applications ranging from scanning a bar code on a package or digitizing a document for display or printing to optical communications systems. Optical sensors generally operate by detecting electromagnetic energy and producing an electrical signal that corresponds to the intensity of the electromagnetic energy striking the optical sensor. Multiple optical sensors are generally used and are often geometrically positioned in arrays with individual optical sensors corresponding to a respective pixel in a resulting electronic display (the term pixel and optical sensor as used in the art and as used in this application are interchangeable). Such arrays allow a larger spatial area to be scanned than could otherwise be performed by a single optical sensor. Other applications may use a raster scan technique in which fewer optical sensors are needed but a document is scanned in an incremental pattern until the document is completely scanned.
An electrical signal from each optical sensor is typically conditioned by an output modifier. The output modifier conditions the electrical signal or converts the electrical signal into an output signal that can be easily understood by a computer processor. The function of the output modifier may be performed by a charge to voltage amplifier or an analog to digital (A/D) converter.
The output signal from the output modifier, corresponding to a respective optical sensor, is next processed in a manner consistent with the specific application. In one application, a computer processor may function as a signal processor that assembles the various output signals and displays or prints the resulting picture. In another application, the computer processor could use the output signal to stop a conveyor belt when groceries have been moved up to the check-out register. The applications in which optical sensors can be used is without bound.
Optical sensors may be manufactured in many semiconductor technologies including MOS (Metal Oxide Semiconductors), CMOS (Complementary MOS), I2L, J-FET, or Bi-CMOS. Each of the manufacturing technologies have trade offs with respect to performance, manufacturing cost, and required associated supplies and interface circuits. Optical sensors have previously been manufactured based on CCD (Charge Coupled Device) technology. Generally CCD's require a dedicated process technology, require multiple supplies, require more complicated interface electronics, and have limited capability for integrating other electronic functions.
Optical sensors generally comprise a photodetector and an electrical circuit. The photodetector produces an electrical signal in proportion to the electromagnetic energy striking the photodetector. The electrical circuit stores an electrical signal produced by the photodetector as an integrated result voltage. An electrical circuit is generally based on either a passive integrator architecture or an active integrator architecture. The passive integrator architecture often comprises a photodiode and a capacitance that includes the capacitance of the photodiode junction, the buffering circuitry, and other parasitic capacitances. The active integrator architecture generally comprises an operational amplifier and an integrating capacitor coupled to a photodiode (See copending U.S. patent application Ser. No. 09/002,904 filed Jan. 5, 1998, now U.S. Pat. No. 6,031,217, entitled Apparatus and Method for Active Integrator Optical Sensors, Attorney Docket No. TI-23303).
An optical sensor generally requires a finite amount of time in which to produce a usable electrical signal in response to electromagnetic energy striking the photodetector. This time period is the integration period and can vary from as little as a few nanoseconds to minutes in duration. At the conclusion of the integration period, the integrated result voltage from the electrical circuit is transferred to the output modifier for further processing. The optical sensor is then reset to zero and a new integration period can begin. The actual time duration of the integration period and the resetting period are generally the same for each optical sensor in the array.
An optical sensor array often comprises multiple photodetectors and an electrical circuit corresponding to each individual photodetector. The optical sensor array may also include a timing circuit that provides a timing sequencing for internal and external operation of the optical sensor array. In addition, the optical sensor array may incorporate an output modifier that conditions the electrical signal into a usable form for a signal processor such as a general purpose computer processor.
Optical sensor arrays generally operate by transferring the electrical signal from each individual optical sensor in the array in sequence to the computer processor through an output modifier. The result is that each optical sensor has a different time frame over which it is integrating and resetting. In applications where the document to be scanned is stationary, the different integration and reset periods would not generally affect the result. However, in applications where the document to be scanned is in motion and a "snapshot" of the document is needed, the different time frames affect the result.
When a document is in motion and passes over the optical array, each optical sensor is at a different point in its integration and reset cycle. At the extremes, one optical sensor will just begin its integration cycle as the document passes in front of the optical sensor. At the other extreme is the optical sensor that has just completed its respective integration cycle as the document passes in front of the optical sensor. The problem is that each optical sensor will thereby integrate an electrical signal that corresponds to a different area of the document. A display of the image from the optical array would show a document that is slanted or "skewed" as a result of the different integration time frames.
The prior art has attenuated the skewing effect by either slowing down the relative motion of the document to the optical array or by speeding up the speed of the optical array. Thus, the speed by which the object is moving past the array is dependent upon the integration speed of the optical sensor.
An optical sensor array based on CCD technology operates fast enough to be used in many applications. A problem with CCDs is that CCDs require special processing, multiple clocks and multiple supplies and are therefore expensive to manufacture.
An optical sensor array manufactured using CMOS technology is generally less expensive. However, a CMOS optical sensor using a passive integration architecture does not generally operate at a sufficient speed to avoid skewing of the document. A CMOS optical sensor using active integrator architecture may operate fast enough to be used in many applications, however, the skewing effect is not completely avoided.